Derivative Instruments - Outstanding Commodity Derivatives (Details)	6 Months Ended
	Jun. 30, 2019 MMBTU $ / MMBTU
Natural gas (IFERC TRANSCO Z6 non-NY)
Derivative [Line Items]
Volume (in Mmbtu) | MMBTU	5,520,000
Swaps Weighted Average (in usd per Mmbtu) | $ / MMBTU	0.41
Natural gas (IFERC TRANSCO Z6 non-NY)
Derivative [Line Items]
Volume (in Mmbtu) | MMBTU	18,450,000
Swaps Weighted Average (in usd per Mmbtu) | $ / MMBTU	2.61
Natural gas (IFERC TRANSCO Leidy Line Receipts)
Derivative [Line Items]
Volume (in Mmbtu) | MMBTU	27,600,000
Swaps Weighted Average (in usd per Mmbtu) | $ / MMBTU	(0.53)
Natural gas (NYMEX)
Derivative [Line Items]
Volume (in Mmbtu) | MMBTU	43,050,000
Swaps Weighted Average (in usd per Mmbtu) | $ / MMBTU	2.85
Natural gas (NYMEX)
Derivative [Line Items]
Volume (in Mmbtu) | MMBTU	55,200,000
Swaps Weighted Average (in usd per Mmbtu) | $ / MMBTU	2.82